1. Field of the Invention
The present invention relates to an NMR (nuclear magnetic resonance) probe and, more particularly, to an NMR probe having a detector portion cooled down to cryogenic temperatures by low-temperature helium gas such that NMR signals are detected with enhanced sensitivity.
2. Description of Related Art
A nuclear magnetic resonance spectrometer is an instrument for observing the NMR signal produced from a sample by applying an intense static magnetic field to the sample such that the magnetic moments of atomic nuclei having nuclear spins within the sample are induced to precess about the static field, then causing the magnetic moments of the atomic nuclei to precess by applying an RF magnetic field perpendicular to the direction of the static magnetic field, and detecting the NMR signal as an RP magnetic field intrinsic to the sample. The NMR signal is emitted when the precessional motion of the magnetic moments of the atomic nuclei returns from excited state to ground state.
The NMR signal is normally quite weak. To enhance the detection sensitivity, piping for low-temperature gas is mounted in the NMR probe having a built-in detector portion. The detector portion is cooled down to cryogenic temperatures, thus reducing thermal noise in the NMR instrument. In this way, the sensitivity of the NMR instrument is enhanced (see Japanese Patent Laid-Open No. 307175/1998, Japanese Patent Laid-Open No. 332801/1998, and Japanese Patent Laid-Open No. 153938/2001).
The positional relation between the prior art NMR probe and a superconducting magnet for producing a static magnetic field is shown in FIG. 1. A main coil B of superconducting wire is wound inside the superconducting magnet A. The main coil B is usually placed within a thermally insulated container (not shown) capable of holding liquid helium or the like, and cooled down to cryogenic temperatures. An NMR probe C is made up of a base portion 40 placed outside the magnet and a cylindrical portion 41 inserted in the magnet. The superconducting magnet A is provided with a cylindrical hole D extending along the central axis. The cylindrical portion 41 is usually inserted into the hole D by moving the cylindrical portion 41 upwardly from the lower opening portion.
The structure of the prior art NMR probe is shown in FIG. 2. A vacuum-insulated container 13 is made up of the base portion 40 placed outside the magnet in conformity with the shape of the NMR probe and a cylindrical portion 41 inserted in the magnet. In the container 13, a cooler 8 is supported by a support post 5 whose one end is mounted to the lower end surface of the container 13. A detector portion 1, made up of a detection coil 2 and a tuning and matching circuit 3, is in thermal contact with and held to the cooler 8. The detection coil 2 is wound along the outer surface of a cylindrical bobbin 15. The center 11 of the detection coil 2 is set at a position where the magnetic homogeneity is optimal within an external static magnetic field produced by a superconducting magnet (not shown).
Thermally insulated piping consisting of a transfer line 7 and a pipe 4 is connected with the cooler 8. The transfer line 7 is used to inject and discharge a low-temperature refrigerant, such as low-temperature helium gas.
A gas pipe 6 for varying the temperature of the sample extends along the center axis of the cylindrical bobbin 15 around which the detection coil 2 is wound. The intersection of the gas pipe 6 and the wall of the vacuum-insulated container 13 is vacuum-sealed by an O-ring 30. In particular, the bobbin 15 is disposed coaxially with the gas pipe 6 used for varying the temperature of the sample. The detection coil 2 is disposed coaxially with the gas pipe 6 and bobbin 15 outside the bobbin 15. Gas used to vary the temperature of the sample is drawn through the gas pipe 6 from bottom to top.
A sample tube 12 holding a sample 14 to be investigated is inserted in the downward direction coaxially with the gas pipe 6 on the inner side of the gas pipe 6 for varying the temperature of the sample such that the center of the sample 14 is coincident with the center of detection 11 of the detection coil 2.
In this structure, the low-temperature refrigerant, such as low-temperature helium gas, is injected into the cooler 8 from the outside through the transfer line 7 and pipe 4, thus cooling both the detection coil 2 and tuning and matching circuit 3 of the detector portion 1. This enhances the Q value of the detection coil 2 and reduces thermal noise in the detection coil 2 and in the tuning and matching circuit 3. This, in turn, improves the sensitivity of the NMR instrument. At the same time, a temperature-controlled gas (variable temperature (VT) gas) is injected into the gas pipe 6 for varying the sample temperature from below to maintain the sample 14 at an appropriate temperature.
The prior art NMR probe cooled down to low temperatures has one problem. As shown in FIG. 3, the center of the detection coil 2 drops to a position 11′ by shrinkage of the support post 5 due to cooling. Therefore, the detection coil 2 deviates from the portion of the highest homogeneity within the static magnetic field. This deteriorates the homogeneity of the static field, impairing the spectral resolution. It is necessary to correct this positional deviation after a lapse of a sufficient time to wait until the inside of the vacuum-insulated container 13 comes to a thermal equilibrium. Consequently, the adjusting work is time consuming and the reproducibility is poor.